Miniscule bag-like animal at the root of human ancestry (and that of starfish, and....)

Luckily it is only the size of a pinhead and has been extinct for some 540 million years, otherwise the monstrous appearance of Saccorhytus coronarius would be the stuff of nightmares. And, like it or not, scientists think that this is one our remote human ancestors.

Category: General

Posted by: Sarah

The millimetre-sized Saccorhytus was a bag shaped creature with a flexible ‘skin’, large mouth and so far as we can tell no anus, which lived in earliest Cambrian times, when life was confined to the seas. At that time life was only just beginning to evolve and split into familiar animal groups of today, such as the sponges, jellyfish-like cnidarians and molluscs. Until now fossil evidence for more complex deuterostome animals, such as sea urchins, starfish, hemichordates and all vertebrates did not appear until between 520 and 510 million years ago.

However, the new fossil discovery of Saccorhytus from early Cambrian age limestones in China has pushed the record of deuterostome evolution back to earliest Cambrian times. Cambridge palaeobiologist, Simon Conway Morris, along with Chinese and German colleagues claim that their new fossil preserves fundamental features characteristic of the deuterostomes, such as bilateral symmetry as opposed to the radial symmetry found in more primitive animals such as corals and jellyfish. As a deuterostome,

Saccorhytus is the earliest known member of this large group and gives us an idea of what our most remote ancestor might have looked like.

The scientists searched through the residues of some 3 tonnes of limestone, which they dissolved in acid, looking for any identifiable organic remains. Organic microfossils of various kinds are known from rocks of this age but most of them are spores and cysts of relatively primitive organisms.

Conway Morris and colleagues have never before seen any fossils quite like Saccorhytus, whose tiny flattened remains are preserved in extraordinary detail. One of the most curious features of Saccorhytus is the array of tiny cones on the surface of the body bag. In the absence of an anus, they may have functioned as return outlets for water taken in through the mouth. As such, they were precursors to the gill-slits, which are one of the defining characters of the deuterostomes.

This new research by Professor Simon Conway Morris of the Department of Earth Sciences in the University of Cambridge is published in Nature. Reference: Jian Han, Simon Conway Morris, Qiang Ou, Degan Shu and Hai Huang. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). DOI: 10.1038/nature21072.

This was the question we were asked by Kanta Kiyomoto a year 8 student at Cambridge International School. Kanta is studying various forms and sources of radiation for his Silver CREST award and wondered if we could help out with some radioactive gems.